This invention relates to air separation.
A well known air separation process comprises compressing a stream of air, pre-purifying the stream of compressed air and cooling it to a temperature suitable for its separation by rectification, subjecting the cooled and purified air stream to a first rectification so as to produce an oxygen-enriched fraction and a nitrogen-enriched fraction, withdrawing an argon-enriched oxygen vapour stream from the first rectification and subjecting it to a second rectification so as to effect a separation as between argon and oxygen and to produce an argon product. The first rectification is typically but not necessarily performed in a double rectification column which comprises a higher pressure rectification column whose top region is in heat exchange relationship with the bottom region of a lower pressure rectification column. The air stream is separated in the higher pressure rectification column into nitrogen vapour and oxygen-enriched liquid air. A feed stream for the lower pressure rectification column is taken from the oxygen-enriched liquid air. The nitrogen vapour is condensed and part of the condensate is used to meet the requirements of the lower pressure rectification column for reflux. The lower pressure rectification column is reboiled by the condensing nitrogen vapour. Oxygen and nitrogen products can therefore be separated in the lower pressure rectification column.
An argon-enriched oxygen vapour stream typically containing from 5 to 15% by volume of argon is withdrawn from an intermediate liquid-vapour contact region of the lower pressure rectification column and introduced into a further rectification column in which the argon is separated. Typically, a crude argon product containing at least 95% by volume of argon and up to about 3% by volume of oxygen with a balance of nitrogen is produced.
Argon and oxygen have similar volatilities. Accordingly, the further rectification column needs to employ quite a large number of distillation plates even to achieve an argon product which is from 95 to 98% pure. It is well known that if one uses conventional distillation plates in the further rectification column it is for practical purposes impossible to reduce the concentration of oxygen in the argon product to less than 10 volumes per million in the further rectification column. Accordingly, in order to produce an argon product of such purity, residual oxygen is conventionally removed by being reacted catalytically with hydrogen to form water vapour, the resulting oxygen-free argon being dried to remove the resulting water vapour and downstream of such drying being further distilled to remove nitrogen and hydrogen impurities.
An improvement to the argon purification process is described in EP-A-0 377 117. In this improvement the further rectification column contains packing in order to effect contact between liquid and vapour. Further, the amount of packing used is sufficient to provide at least 150 theoretical plates in the further rectification column. It is reported in EP-A-377 117 that by employing approximately 180 theoretical plates an oxygen content of less than 1 volume per million in the crude argon product of the further rectification column can be achieved with an economically acceptable argon yield.
EP-A-377 117 further discloses separating nitrogen from the crude argon in the yet further rectification column so as to produce a pure argon product. As disclosed in EP-A 0 520 382, however, very low levels of nitrogen can be achieved in the argon product without resort to this yet further rectification column. Firstly, the nitrogen concentration of the argon-enriched oxygen vapour to the further rectification column can be kept below 50 volumes per million. Secondly, the further rectification column may include an argon-nitrogen separation section above the level of the argon product outlet. Accordingly, the argon product may include less than 10 volumes per million of nitrogen. Thus, if at least 150 theoretical plates are used in the further rectification column (excluding the argon-nitrogen separation) no further purification of the argon product will typically need to be performed.
The key advantage of these improvements is that they eliminate the need for hydrogen, a highly inflammable and explosive gas, to be employed in the vicinity of the air separation plant. On the other hand, the large number of theoretical plates (at least 150) results in an exceptionally tall rectification column. Indeed, in practice, the total height of the column and surrounding thermal insulation has been known to exceed 70 meters.
There is thus a need to provide a method and apparatus for producing argon by rectification which offers the advantage of making the use of hydrogen purification unnecessary but which makes possible the use of shorter distillation columns than those required to produce an argon product of given oxygen purity. The invention aims at meeting this need.